A polymeric porous material having a large number of intercommunicable micropores is utilized in various fields, for example, as separation films to be used to produce ultrapure water, purify chemicals, and treat water; a waterproof moisture-permeable film to be used for clothes and sanitary materials; and the separator for use in the battery.
A secondary battery is widely used as the power source of OA, FA, household appliances, and portable apparatuses such as communication instruments. A lithium-ion secondary battery has a favorable volumetric efficiency when it is mounted on apparatuses and allows the apparatuses to be compact and lightweight. Therefore there is an increase in the use of portable devices in which the lithium-ion secondary battery is used. Owing to research and development of a large secondary battery which has been made in the field of load leveling, UPS, an electric vehicle, and in many fields relating to the problem of energy and environment, the lithium-ion secondary battery which is a kind of a nonaqueous electrolytic solution secondary battery has widely spread in its use because the lithium-ion secondary battery has a large capacity, a high output, a high voltage, and an excellent long-term storage stability.
The lithium-ion secondary battery is so designed that the upper limit of the working voltage thereof is usually 4.1V to 4.2V. Because electrolysis occurs in an aqueous solution at such a high voltage, the aqueous solution cannot be used as an electrolyte. Therefore as an electrolytic solution capable of withstanding a high voltage, a so-called nonaqueous electrolytic solution in which an organic solvent is used is adopted. As a solvent for the nonaqueous electrolytic solution, an organic solvent having a high permittivity which allows a large number of lithium ions to be present is widely used. An organic carbonate ester compound such as polypropylene carbonate or ethylene carbonate is mainly used as the organic solvent having a high permittivity. As a supporting electrolyte serving as the ion source of the lithium ion in the solvent, an electrolyte having a high reactivity such as lithium phosphate hexafluoride is used in the solvent by dissolving it therein.
The separator is interposed between the positive electrode of the lithium-ion secondary battery and its negative electrode to prevent an internal short circuit from occurring. Needless to say, the separator is demanded to have insulating performance as its role. In addition the separator is required to have a porous structure so that air permeability of allowing lithium ions to pass therethrough and a function of diffusing and holding the electrolytic solution are imparted to the separator. To satisfy these demands, a porous film is used for the separator.
Because batteries having a high capacity are used recently, the degree of importance for the safety of the battery has increased. A shut-down property (hereinafter referred to as SD property) contributes to the safety of the separator for the battery. The SD property is the function of preventing the temperature inside the battery from rising owing to closing of micropores when the battery has a high temperature of 100° C. to 150° C., which leads to shut-off of ion conduction inside the battery off. The lowest temperature of temperatures at which the micropores of a multilayer porous film are closed is called a shut-down temperature (hereinafter referred to as SD temperature). To use the multilayer porous film as the separator for the battery, it is necessary for the multilayer porous film to have the SD property.
Because the lithium-ion secondary battery has become higher in its energy density and power in recent years, the normal shut-down property does not sufficiently work. Thus the temperature inside the battery rises over 150° C. which is the melting point of PE and continues to rise. As a result, a short circuit occurs between the positive and negative electrodes owing to breakage of the separator caused by thermal contraction, which may lead to the occurrence of ignition. Thus to secure safety, the separator is demanded to have a higher degree of heat resistance than the degree of heat resistance to be obtained by the present SD property.
To comply with the above-described demand, there are proposed in the following patent documents 1, 2, and 3 the multilayer porous films each having the porous layer, containing the filler and the resin binder, which is laminated on at least one surface of porous polyolefin resin film. It is described in these patent documents that the methods of producing the multilayer porous films are excellent in safety because in these multilayer porous films, by forming the coating layer containing the inorganic filler or the like at a high content rate on the porous film, it is possible to prevent the occurrence of a short circuit between the positive and negative electrodes, even though abnormal heat is generated and the temperature of a battery continues to rise over the SD temperature.